EFFECTS OF REGIONS OF VISIBLE SPECTRUM 



781 



and minimum reflection from leaves occurred in the blue end of the s))ec- 

 trum. Reflection in the region between 4500 and 4800 A (blue), averaged 

 around 5.5 per cent, while between 6200 and 7100 A (red) it averaged 

 around 6.5 per cent. Pokrowski found that the lower surfaces of leaves 

 reflected more radiation at all wave-lengths than did the upper surfaces. 

 The percentage of reflection of leaves decreased with the age of the leaf in 

 Tilia parviflora. 



Table 4. — Reflection, Transmission, and Absorption of Leaves as 



Found by Pokrowski 

 (Total incident radiation = 1) 



Pokrowski's studies of reflection from leaf surfaces have been, in 

 general, verified by Shull (44) and Hibben (16), both of whom, working 

 independently, used a spectrophotometer and compared the reflection 

 from leaf surfaces with that from magnesium carbonate. Shull, using a 

 wide variety of leaves, obtained maximum reflection of green leaves in the 



o 



region 5400 to 5600 A, which amounted to 6 to 8 per cent from dark green 

 leaves and 20 to 25 per cent from light green ones. The amount of 

 reflection decreased with increasing age of the leaf until the chlorophyll 

 concentration reached its maximum and remained constant from that 

 time until autumn. Hibben reported maximum reflection in the region 



o o 



5500 to 5600 A, reaching 28 per cent at wave-length 5600 A, with 



o 



secondary increases in the blue (4400 to 4600 A) and in the red (6750 to 



o 



7000 A). Both Shull and Hibben found, like Pokrowski, that the lower 

 surfaces of leaves reflected more light than the upper surfaces. Leaves 

 with white surfaces, according to Shull, reflect all wave-lengths uniformly, 

 regardless of color. With the disappearance of chlorophyll and the 

 appearance of autumn colors, maximum reflection shifts to the red and 

 yellow. Thus, yellow birch leaves were found by Shull to reflect 42 per 

 cent of the incident radiation from the upper surface, with a maximum in 



o 



the red at 6600 A. With less completely yellowed poplar leaves he 

 obtained the same total percentage reflection, but the maximum was in 



o 



the yellow at 5800 A. Hibben reports maximum reflection from autumn 

 leaves in the region 6000 to 7000 A and greatest amount of reflection from 

 the most brilliantly colored leaves. Shull, unlike Pokrowski, found in 

 a number of the green leaves tested a decrease in the percentage of reflec- 



